Conventionally known communication system is that abase unit and a plurality of communication terminals (subbase units) are connected to a communication line (transmission line), to thereby perform communication between each communication terminal and the base unit. As an example of this kind of communication system, there is a system in which the base unit regularly monitors a state of the communication terminal, and when there is a change in the state of the communication terminal, a signal is transmitted from the base unit to other communication terminal so as to respond to the change of the state (see Japanese Patent Publication No. 1180690, Japanese Patent Publication No. 1195362, and Japanese Patent Publication No. 1144477). In this kind of communication system, for example when on/off state of a switch appended to the communication terminal is changed as the state change of the communication terminal, the base unit turns on/off an illumination apparatus and an air conditioner connected to other communication terminal, as a processing for responding to the state change. In this communication system, as shown in FIG. 25, communication terminals 1 don't perform communication directly but always perform communication via a base unit 2.
However, in the above-described structure, the communication terminals 1 always perform communication via the base unit 2, and therefore a communication speed is slow for performing polling by the base unit 2 directed to the communication terminals 1. Accordingly, the communication system with this structure is unsuitable for transmitting information with relatively large volume of data such as analog data (continuous data of electric energy). Further, in the communication system with this structure, an overall system is halted at the time of a failure, etc., of the base unit 2, thus involving a problem that reliability is low.
Meanwhile, there is also proposed a communication system configured to directly perform communication by communication terminals connected to a communication line in a peer-to-peer mode (called P2P hereafter), capable of transmitting the information with relatively large volume of data by improving the communication speed. Note that in this communication system, power is supplied to a plurality of communication terminals by one power feeder connected to the communication line.
The communication system of directly performing communication with communication terminals as described in the latter communication system, is desirable from a viewpoint of the communication speed and reliability. However, the former communication system in which communication is performed by the communication terminals 1 via the base unit 2 is widely spread. Therefore, in order to effectively utilize an already established communication system, the communication system as shown in FIG. 26 can be considered, which is the communication system of a coexistence of the former communication system, and the latter communication system capable of performing high-speed communication (see Japanese Patent Application Laid-Open No. H8-274742)
In the communication system of FIG. 26, a first communication terminal 1 performs communication via the base unit 2 by using a first protocol signal (voltage signal). Meanwhile, a second communication terminal 10 performs direct communication by using a second protocol signal (voltage signal) which is superimposed on the first protocol signal in a manner of synchronizing with the first protocol signal. The second protocol signal has a higher frequency than the frequency of the first protocol signal, and there is a difference in a signal level, etc., between the first protocol signal and the second protocol signal. Therefore, the first communication terminal 1 and the second communication terminal 10 cannot perform communication with each other, although they are connected to the same communication line 4.
Incidentally, in the aforementioned communication system, a plurality of terminals (base unit 2, first communication terminal 1, and second communication terminal 10) are connected to the two-wire type communication line 4. Therefore, an input impedance of each terminal is connected in parallel via the communication line 4. Accordingly, in order to perform communication by using the voltage signal (second protocol signal) between second communication terminals 10, the impedance of the terminal connected to the communication line 4 is required to be increased. However, the terminal with low impedance also exists in the already established terminals. Particularly, the base unit 2 also functions as a power supply and includes a smoothing capacitor in an output stage, thus showing relatively low impedance. Such a terminal is required to have high input impedance to a signal component, by connecting a high impedance module (not shown) between the terminal and the communication line 4, to thereby provide high impedance to the second protocol signal.
However, when the already established communication system is utilized, a contractor needs to connect the high impedance module between the terminal with low impedance of the aforementioned already established terminals, and the communication line 4, to thereby perform communication between the second communication terminals 10 by using the voltage signal. Therefore, when the second communication terminals 10 are introduced, the contractor needs to confirm a position of the already established terminal to perform a work of connecting the high impedance module. If the contractor forgets to connect the high impedance module, there is a possibility that the impedance of the communication line 4 is reduced, and the communication between the second communication terminals 10 is not established.
As described above, there is a problem in introducing the aforementioned communication system, such that it is time-consuming and laborious to examine a situation of a work site, thus imposing a large load on the contractor, and therefore the aforementioned communication system cannot be easily introduced.